Gas turbine engines are known to include a compressor for compressing air, a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. Gas turbine engines using annular combustion systems typically include a plurality of individual burners disposed in a ring about an axial centerline for providing a mixture of fuel and air to an annular combustion chamber disposed upstream of the annular turbine inlet vanes. Other gas turbines use can-annular combustors wherein individual burner cans feed hot combustion gas into respective individual portions of the arc of the turbine inlet vanes. Each can includes a plurality of main burners disposed in a ring around a central pilot burner.
During operation, the combustion flame can generate combustion oscillations, also known as combustion dynamics. Combustion oscillations in general are acoustic oscillations which are excited by the combustion itself. The frequency of the combustion oscillations is influenced by an interaction of the combustion flame with the structure surrounding the combustion flame. Since the structure of the combustor surrounding the combustion flame is often complicated, and varies from one combustor to another, and because the combustion flame itself may vary over time, it is difficult to predict the frequency at which combustion oscillations occur. As a result, combustion oscillations may be monitored during operation and parameters may be adjusted in order to influence the interaction of the combustion flame with its environment.
A combustion flame emits sound energy during combustion. A more uniform flame will generate more uniform acoustics, but perhaps with higher peak amplitude at a particular frequency than a less uniform flame. When an emitted frequency of combustion coincides with a resonant frequency of the combustion chamber the system may operate in resonance, and the resulting combustion dynamics may damage the gas turbine components, or at least reduce their lifespan.
One known way to reduce the interaction of the combustion flame with the combustion acoustics is to reduce the coherence of the flame, i.e. reduce the spatio-temporal uniformity of the flame. A flame with less uniform combustion throughout its volume is likely to perturb the gas turbine less than a uniform flame because the energy released is spatially distributed and therefore decreases its coupling to the system resonant frequencies or acoustic modes. This is the well known Rayleigh criterion. As a result, combustion dynamics of flames with less uniform combustion throughout its volume are less likely to be exacerbated than by a more uniform flame.
One way that has been utilized to reduce flame coherence has been to vary the fuel/air ratio throughout the flame. Main premix burners often have a swirler that swirls an airflow flowing through the burner. Fuel outlets in the burner introduce a flow of fuel into the airflow to produce a fuel/air mixture of a certain ratio. The fuel/air ratio from main burners may be varied. For example, some of the main burners of a combustor may be controlled by one fuel stage, and the remaining burners of the combustor by another stage. Since the structure of the main burners and swirlers in them are uniform throughout the burners in the combustor, varying the fuel from burner to burner varies the fuel/air ratio. Since each fuel/airflow has a different amount of fuel when it reaches the combustion flame, the combustion/temperature of the combustion flame varies throughout its volume and the flame is less coherent.
Such a fuel biasing of the combustion flame has drawbacks. Separate fuel stages are very expensive to manufacture and complicated to operate. Further, localized regions of leaner and richer combustion within the combustion flame produce less than optimal emissions.
Another way that has been utilized to reduce flame coherence has been to vary portions of the combustion flame axially with respect to other portions of the combustion flame which results in a less uniform combustion flame, thereby reducing combustion dynamics. This has been accomplished, in one example, by increasing the volume of fuel/air flow through one burner with respect to another burner. This has also been accomplished by positioning burners in different locations axially with respect to other burners in a combustor. However, these configurations may not work under all situations, so there remains room in the art for combustor configurations to reduce flame coherence and associated combustion instabilities.